Nevertheless, manufacturing GaN single-crystal substrates is not a simple matter. Inasmuch as heating solid GaN does not liquefy it, crystals cannot be formed by the ordinary Bridgman or Czochralski methods that grow a crystal from a melt. Granted that applying ultra-high pressure to and heating GaN can make it into a melt, doing so is difficult nonetheless, and the fact that large-scale crystals cannot thus be made is obvious.
GaN layers and InGaN layers of some 1 μm, or 1 μm or less, fabricated onto a sapphire substrate are currently being fashioned by vapor-phase deposition techniques, which include HVPE, MOC, AND MOCVD. Efforts are being made to produce thick GaN crystal by vapor-phase deposition techniques such as these for growing thin films.
However, being that these originally are technologies for depositing thin films 1 μm and under onto sapphire substrates, they are in the first place notorious for incidents of defects. If nothing but LEDs are fashioned onto a sapphire substrate, since the GaN layer would be thin the stress would be small; but with stress being stepped up, increasing defects and distortion, in the film as the thickness is built up in order to produce bulk crystal, and with it peeling off from the substrate, thick material cannot be obtained.
A. Epitaxial Lateral Overgrowth
In this regard, a technique called epitaxial lateral overgrowth (ELO) has been devised. Literature on the subject includes, for example:                1) Akira Sakai, and Akira Usui. Reduction of dislocation density in GaN films by epitaxial lateral overgrowth. OYO BUTSURI (A monthly publication of The Japan Society of Applied Physics), vol. 68, No. 7 (1999), p. 774.        2) Akira Usui. Thick Layer Growth of GaN by Hydride Vapor Phase Epitaxy. Transactions of the Institute of Electronics, Information and Communication Engineers, C-11, vol. J81-C-11, No. 1 (1998), pp. 58-64.        3) Kensaku Motoki et al. Preparation of Large Freestanding GaN Substrates by Hydride Vapor Phase Epitaxy Using GaAs as a Starting Substrate. Jpn. J. Appl. Phys., Vol. 40 (2001), pp. L140-L143.        4) Japanese Unexamined Pat. App. Pub. No. 2000-22212. GaN Single-Crystal Substrate and Method of Its Manufacture.        5) Japanese Unexamined Pat. App. Pub. No. 2000-12900. GaN Single-Crystal Substrate and Method of Its Manufacture.        
ELO is a method of decreasing dislocations by: covering a planar substrate surface over its entirety with equilateral triangles whose sides are L (several μm); putting a mask F, perforated with windows (size E) positioned to fit the vertices of the equilateral triangles, on the substrate; and growing GaN through the top of the mask. Mask shapes are illustrated in FIGS. 1 and 2. In FIG. 1, round windows are iterated in an array according to the pattern of the equilateral triangle vertices. In FIG. 2, regular-hexagonal windows are iterated in an array according to the pattern of the equilateral triangle vertices. The ELO mask F includes the portion 3 that covers, and the windows 4, while the contour of the windows 4 can be of all sorts, such as round, polygonal, stripes, etc. A mask thus in which an equilateral-triangle window pattern is repeated is formed onto a sapphire substrate, and GaN is vapor-phase deposited through the top of the mask. The mask material is a substance—SiN or SiO2—which GaN does not grow.
An ELO film-deposition procedure is represented in FIG. 3. FIG. 3(1) depicts a sectional view of a situation in which the ELO mask F having the covering portion 3 and windows 4 has been formed on top of an under-substrate 2. FIG. 3(2) depicts a situation in which GaN crystal seeds 5 have been deposited on the windows 4 by vapor-phase deposition. Tiny crystal seeds grow on the substrate surface in the isolated windows. The crystal directions are determined so as to coordinate with the substrate. Atop the mask crystal seeds are not produced. The material of the ELO mask acts to suppress GaN growth. As growth proceeds the isolated crystal seeds gradually bulge and coalesce, turning into islands. The islands join and take on a thin-film form. The window interiors eventually become covered with a GaN thin-film of even membrane thickness. FIG. 3(3) depicts a situation in which the GaN has grown into a thin film 6. Because the islands coalesce their boundaries turn into complex crystalline defects. While high-density defects of this sort are produced, along with the film growth the defects stretch upward as they are. This forms dislocations extending in the perpendicular direction. The dislocations stretch along as they are, without diminishing. This is kept up since there are high-density dislocations from the first.
Having grown to the height of the mask the GaN thin film grows further, higher than the mask; but because GaN does not grow on the mask, GaN crystal 7 starts to protrude in a bell shape, as illustrated in FIG. 3(4). The sloping faces 23 are called facets. The bottom faces of the facets 23 are equivalent planes of indices {1-101}, {2-1-12}. The bell-shaped formations advance and the GaN film grows, filling out into deposition-film windows through neighboring windows and taking on pyramid shapes 8. The dislocations 20 stretch heading upward identically with the growth direction. This is illustrated in FIG. 3(5). Once the GaN becomes pyramidal, that form is sustained, and since it can extend no further upward, the GaN film rides onto the mask. The facets 24 at this point are referred to as “critical facets.” Now the growth proceeds sideways along the top of the mask, with the facets being sustained. The dislocations bend 90° at the critical facets 24, becoming transverse dislocations 22. By bending over, the dislocations decrease at this time. GaN trapezoidal-pyramid crystal in a situation of this sort is illustrated in FIG. 3(6).
The thin-film growth advances horizontally in the form of regular hexagonal pyramids, inasmuch as there are 6 equal facets. Since six lateral facets are produced, in actuality the growing crystal turns into trapezoidal hexahedrons and spreads. When the growth atop the mask proceeds, the lumps of GaN crystal that have grown through neighboring windows come into contact in a dashed line 25 perpendicular to the windows (FIG. 3(7)). Thereafter the GaN crystal grows to fill in the channels bordering the regular hexahedrons. The border channels 26 become filled. The dislocations 22 having extended across from either side collide in the border channels 26, and the majority stops there. When the crystal growing through neighboring windows coalesces and the outer surface becomes flat, as illustrated in FIG. 3(8), the direction of growth switches upward once more. This means that the growth direction switches twice. Although the direction in which the dislocations 22 extend again changes to upward, most of them cancel each other out. The fact that the growth heads upward subsequent to reduction in the dislocation density means that GaN crystal in which dislocations are comparatively few is produced. Those are the essentials of ELO.
One more advance technology in connection with growing GaN and which forms a basis for the present invention is not a well-known public technology as is ELO: It has been created by the inventors of the present invention, but not yet announced. Particles of a metal and a dielectric are. placed atop a substrate, over which shut-off, defect-gathering regions (closed defect-gathering regions H) are formed; and surrounding them concentrically, low-dislocation concomitant single-crystal regions Z where defects are few and the electro-conductivity is high, are formed; and in the intervening gaps, low-dislocation remaining single-crystal regions Y are formed. Once they have formed dislocations do not disappear, but because they are absorbed by the closed defect-gathering regions H, dislocations in the other regions—the low-dislocation concomitant single-crystal regions Z and the low-dislocation remaining single-crystal regions Y—decrease.
These regions as such cannot be seen with an SEM nor a TEM; the low-dislocation concomitant single-crystal regions Z and the low-dislocation remaining single-crystal regions Y can be distinguished and viewed by means of CL (cathode luminescence). FIG. 4 represents a plan view of a situation in which defect-seeding masks X have been disposed atop an under-substrate 2. These are round-patterned from a dielectric such as a high melting-point metal, SiO2, or SiN. They are also situated in the vertex positions in a pattern of iterated equilateral triangles. The period and diameter of the defect-seeding masks X (period M, diameter B), however, are far larger than those of the mask F (period L, size E) for ELO (M>>L; B>>E). A seeding-mask technique will be explained according to FIG. 5. FIG. 5(1) represents the sapphire under-substrate 2. FIG. 5(2) represents a situation in which a uniform GaN buffer layer 52 has been formed onto the sapphire under-substrate 2. FIG. 5(3) is a sectional view of a situation in which the defect-seeding masks X, which have a growth-suppressing action, have been set atop the GaN buffer layer 52.
FIG. 5(4) represents a situation in which GaN has been grown onto the buffer layer 52 and seeding masks X by vapor-phase deposition. The closed defect-gathering regions H grow atop the defect-seeding masks X. Surrounding them the low-dislocation concomitant single-crystal regions Z, which have facets 53, grow. The low-dislocation remaining single-crystal regions Y grow along flat faces 54 on the boundaries. The technique thus yields a crystal in which defects are localized into the closed defect-gathering regions. A single crystal as an entirety, with the defects being localized into the closed defect-gathering regions, the leftover portions (Y and Z) of the crystal turn out to be low-dislocation, low-defect. FIG. 5(5) represents a subsequent situation in which a GaN substrate having a flat surface has been obtained by shaving away the reverse-side under-substrate material and planarizing the product. FIG. 6 represents a microscopic observation by CL (cathode luminescence). The disk-shaped areas where the low-dislocation concomitant single-crystal regions Z are appear dark and are therefore readily apparent. Without CL, since these would be transparent viewed under a microscope, they could not be discerned.
In early-stage growth with an ELO mask the dislocations within the GaN decrease. They do so in that the dislocations diminish by canceling each other out and thus are in actuality reduced. The defect-seeding mask method (not publicly known) decreases dislocations in the leftover regions by concentrating defects in mid-stage growth into the closed defect-gathering regions H.